1. Field of the Invention
The present invention relates, in general, to longitudinal magnetic field compacting methods and devices for manufacturing high performance rare earth sintered magnets having butterfly shapes for use in VCM (Voice Coil Motor) of HDD (Hard Disk Drive) or DVD (Digital Versatile Disk), disk or coin shapes for use in coreless motors, and block shapes for use in linear motors.
More particularly, the present invention is directed to a longitudinal magnetic field compacting method and device for manufacturing neodymium (Nd) based rare earth sintered magnets, characterized in that a longitudinal compacting process is used under a pulse magnetic field so that rare earth powders are oriented in a direction of an applied magnetic field, whereby the rare earth sintered magnet can be fabricated in the shape of a butterfly for VCM of HDD or DVD and a disk or coin for coreless motors with superior magnetic properties, as well as a block for linear motors. Further, compared to conventional longitudinal compacting methods using a static magnetic field, a compacted body of the present invention has the same shape as end products, and there is no additional processing cost, thereby lowering manufacturing costs. In addition, the rare earth powders can be subjected to an aligning process and a longitudinal compacting process at the same time under the high pulse magnetic field of 50–70 kOe. Thereby, the resulting rare earth magnet can have magnetic properties of 42–50 MGOe better than those fabricated by conventional transverse static magnetic field compacting methods. Consequently, the longitudinal compacting method and device of the present invention can be effectively used, therefore realizing high practical applicability.
2. Description of the Related Art
With great advances in magnetic techniques, there have been required rare earth sintered magnets in the shape of a butterfly for use in VCM of HDD or DVD, a disk or coin for use in coreless motors, and a block for use in linear motors, with a maximum magnetic energy product of 40–49 MGOe.
In order to manufacture the rare earth magnet with excellent magnetic properties (maximum magnetic energy product), an aligning process and a magnetic field compacting process of rare earth powders in a direction of an applied magnetic field should be improved. Examples of conventionally used magnetic field compacting methods include a longitudinal compacting method and a transverse compacting method both using a static magnetic field.
As for such a longitudinal static magnetic field compacting method, rare earth powders having a particle size of 2–6 μm are packed in a metal mold having a cavity with a predetermined shape, to which the static magnetic field of 10–20 kOe is applied, thus aligning the powders in the direction of an applied magnetic field (anisotropic). Then, a direction of an applied compacting pressure is applied to be coincident with the direction of the applied magnetic field. In such a case, the alignment of the rare earth powders is performed by generating a static magnetic field with the use of an electromagnet, which is fabricated by winding a coil around an iron core. However, electromagnets have limitations in that the strength of the magnetic field has a maximum of 30 koe. Accordingly, the conventional longitudinal compacting method using the static magnetic field is disadvantageous in terms of the fabrication of the magnet with a degree of orientation of 89%. As such, the value of the maximum magnetic energy product, which is in proportion to product of such a degree of orientation, is 42 MGOe. Consequently, the magnet fabricated by the longitudinal static magnetic field compacting method suffers from relatively low magnetic properties.
In addition, in the case of the transverse static magnetic field compacting method, the direction of an applied compacting pressure is perpendicular to the direction of the applied magnetic field. Upon the transverse compacting, the degree of orientation of the powders is increased to 93%, thus obtaining the magnetic properties of 46 GMOe. However, it is impossible to compact the rare earth powders to butterfly-shaped, and disk- or coin-shaped magnets with superior magnetic properties of 42 GMOe or higher. Hence, the rare earth powders are compacted and sintered in the shape of a block or arc, and then processed to the desired shape of end products. Therefore, manufacturing costs increase.
As a result, limitations are imposed on the efficiency of conventional longitudinal and transverse compacting methods using a static magnetic field, and thus practical applicability thereof is minimized.